Techniques for obtaining information about the structure of biological membranes by electron microscopy are being investigated. A high resolution scanning transmission electron microscope with 3 angstroms resolution and a liquid helium cold stage will allow microscopy of unfixed, unstained membranes at atomic resolution. Chemically specific heavy element labels are used as structure markers. One such label currently being used is a complex of platinum with glycylmethionine. It binds to bacteriorhodopsin in a stoichiometry of about 3 Pt atoms per protein molecule. Membranes containing bacteriorhodopsin labelled by Pt will be viewed in the STEM to assay the visibility and possible beam-damage related movement of the labels. The effect of temperature on the motion of heavy element atoms will be investigated in several preparations. Electron diffraction patterns of the labelled bacteriorhodopsin membranes contain information to at least 3.5 Angstroms. An attempt to determine the structure of bacteriorhodopsin at high resolution from the diffraction patterns is being made. At present, the cold stage is under construction, the diffraction patterns have been recorded and analysis is beginning, and biochemical characterization of the Pt binding is in progress.